STEPS OF CONSTRUCTION
(i) Draw a ray OA.
(ii) With O as centre and a convenient radius, draw a long arc, cutting OA at P.
(iii) With as centre and the same radius as above, draw an arc, cutting the previous arc at Q.
(iv) With Q as centre and the same radius, draw yet another arc, cutting the arc drawn in Step (ii) at R.
(v) Join OR and produce it to a point B.
Then, `angleAOB` is the required angle of measure `120^(@)`.
